1. Field of the Invention
The present invention generally relates to the structure of anode foils suitable for construction of electrolytic capacitors and the manufacturing thereof. More particularly, the present invention relates to manufacturing systems and processes for creating a porous anode foil for use in an electrolytic capacitor.
2. Background Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices such as an implantable cardiac therapy device (“ICTD”). These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
ICDs typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an ICD may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with an electrolyte solution. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical electrolyte solution may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the magnitude of the capacitance of an aluminum electrolytic capacitor is determined largely by the surface area of the anodes, one approach that may be used to increase the energy density in the capacitor is to minimize the volume taken up by the paper and the cathode thereby permitting the number of anodes to be increased or maximized. A multiple anode stack configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the overall size of such an electrolytic capacitor. A multiple anode stack consists of a number of units, each in turn consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. Energy storage density can be increased by using a multiple anode stack configuration element.
In fabricating anode foils for use in an electrolytic capacitor having a multiple anode stack configuration, a physical mask is conventionally used to mask areas during the etching process in order provide unetched tabs, which in turn are used during welding processes. Without the unetched tab areas, welds will not appropriately form the connections between the anodes in a stack configuration. In a conventional process, the physical mask is typically held no less than 1/32 inch from the foil to create the necessary unetched area. The tab contacting the foil must be created such that a tapered angle from the top of the tab to the tab connecting to the surface is made. The tapered angle allows the transition from unetched to etched area to be less abrupt by tapering the current density. Without the taper, the foil is susceptible to cracking along the transition edge due to a high current density attack.
Conventional methods of preparing anode foils for use in electrolytic capacitors have a variety of constraints and problems which result in reduced efficiency and increased manufacturing costs.
What is needed are improved high capacity anode foils suitable for electrolytic capacitors and methods of making such anode foils.